Smith attended Saint Cuthbert's High School in St Helens. When he was 15 he fell off his bike on the way to school breaking his hip which required him to use crutches for 16 weeks. During that period Smith began playing darts for the first time out of boredom and threw his first 180 whilst on crutches. After leaving school he took a joinery course at college but dropped out with one exam remaining to play in a darts tournament instead a decision which he has described as the best of his life. He was a regular winner of local darts events and made his debut in a PDC major at the 2009 UK Open where he lost 6–5 to Dave Ladley in the first round.
Smith reached the last 32 of the 2010 UK Open, knocking out Peter Manley and Matt Clark before losing to Mervyn King. He was one of a number of young players to make a significant impact in the tournament, along with William O'Connor, Arron Monk and Reece Robinson.
He was the number four seed for the 2011 PDC Under-21 World Championship, and lost in the quarter-finals to Benito van de Pas.
On 26 February 2011, Smith hit a nine-dart finish in a PDC Youth Tour event in Barnsley, against Michael van Gerwen. He eventually won through to the final, where he lost to Shaun Griffiths. The following day, he won his first PDC Pro Tour event, defeating Dave Chisnall 6–5 in the final of the 2011 UK Open Qualifier 2.
Smith had now become a full-time professional on the PDC darts Circuit and made his debut at the World Championships in 2012, losing 0–3 to Co Stompé in the first round. Smith only hit 3 of his 14 darts at a double during the match.
Smith won his second professional tournament in January 2012, at the Players Championship Event 2 in Benidorm. He won seven matches, concluding with a 6–3 victory over Justin Pipe to take the title and the £6,000 prize money. In April, he earned a place in the European Tour Event 1 in Vienna by defeating Les Wallace and Shaun Griffiths in the UK qualifier. He played Jamie Caven in the first round and hit a nine dart finish on his way to a 6–4 win, but then lost 5–6 to Mark Walsh in round two. His results meant he qualified for the World Matchplay for the first time in his career via the ProTour Order of Merit. He faced Raymond van Barneveld in the first round and was comfortably beaten 4–10. Smith also played in the World Grand Prix for the first time, losing to Phil Taylor 0–2 in sets in the first round. After all 33 ProTour events of 2012 had been played, Smith was 25th on the Order of Merit, inside the top 32 who qualified for the Players Championship Finals. It was Smith's first appearance in the tournament and he edged out Ian White 6–5 in the first round, before losing 6–10 to Kim Huybrechts.
Smith's position on the ProTour Order of Merit also saw him qualify for the 2013 World Championship, as he took the second of sixteen places that were awarded to the highest non-qualified players. However, he ran into an in-form Raymond van Barneveld in the first round who averaged 108.31 as Smith could only win one leg during the match in a 0–3 defeat. Smith reached the final of the third UK Open Qualifier in March, but was beaten 2–6 by Michael van Gerwen. Smith won the 2013 PDC World Youth Championship with a 6–1 victory over Ricky Evans which was broadcast just before the 2013 Premier League Darts Final. Smith led van Barneveld 3–0 in the third round of the UK Open, but went on to lose 8–9. In October, he advanced to the semi-finals for the second time in 2013 at the 12th Players Championship but his run was ended with a 6–3 defeat to Kevin Painter. His youth title earned him a place in the Grand Slam of Darts and he produced an impressive display in his first group match against Dave Chisnall as from 3–1 down he hit legs of 11, 12, 14 and 13 darts to win 5–3 with an average of 103.17. However, he then lost 5–2 to Scott Waites and 5–1 to Ted Hankey to finish third in Group H and be eliminated from the tournament.
Smith broke into the world's top 32 on the Order of Merit just before the cut-off for the 2014 World Championship to qualify through the main ranking list for the first time. He beat Morihiro Hashimoto 3–1 in the first round. He then incredibly defeated reigning and 16-time world champion Phil Taylor 4–3 in the second round, taking out the bullseye on a 128 finish to win the match. In the next round Smith hit a 136 finish, with his opponent Peter Wright waiting on 41, to lead 3–2 but only won one more leg after this to lose 4–3 in a high quality encounter. He was named the PDC's Young Player of the Year at the Annual Awards in January. Smith won through to the final of the sixth UK Open Qualifier of the year but averaged just 70.94 as he was whitewashed 6–0 by Michael van Gerwen. A rematch against Wright followed in the third round of the UK Open with Smith throwing for the match in the final leg, but he missed a total of seven darts at doubles to lose 9–8. His second final of 2014 was at the fifth Players Championship which he reached by beating Raymond van Barneveld 6–3 in the quarter-finals and Gary Anderson 6–4 in the semis, but was again whitewashed in the decider, this time by Brendan Dolan.
At the World Matchplay he recorded a 10–8 victory over Justin Pipe, before losing 13–6 to Taylor in the second round. At the European Darts Grand Prix, Smith eliminated Gary Anderson, Adrian Lewis and Stephen Bunting en route to facing Mervyn King in the final. Smith paid the price for missing too many doubles throughout the match including one to claim the title as he lost his third ranking final of the year 6–5. Another final followed a week later at the 14th Players Championship where he lost 6–4 to Van Gerwen. Smith's rich vein of form continued into the European Darts Trophy as he reached the final and came back from 3–0 against Van Gerwen to win the title 6–5, which included a key 158 finish to break throw in the ninth leg. He suffered first round exits at the World Grand Prix and European Championship, but topped Group F at the Grand Slam by winning all three of his games. Smith then knocked out Christian Kist 10–5, but was thrashed 16–3 by Taylor in the quarter-finals losing 13 straight legs from 3–3. His year in the major events concluded with a 6–4 defeat to Andy Smith in the opening round of the Players Championship Finals.
After seeing off Mensur Suljović 3–1 in the first round of the 2015 World Championship, Smith overcame Brendan Dolan 4–2 after having been 2–0 down, to set up a third round tie with fellow St. Helens player Stephen Bunting. Despite taking out checkouts of 170 and 132, Smith was 3–0 down, before finding his high scoring game to restrict Bunting to one leg as he closed the gap to 3–2. Ultimately his slow start to the match proved costly as he lost the next set to be knocked out 4–2 in a high quality game which saw each player average 102 and hit eight 180s apiece. Smith won the fifth UK Open Qualifier by beating Adrian Lewis 6–5. A rematch with Bunting followed in the third round of the UK Open, with Smith missing two match darts in the deciding leg to be beaten 9–8. He claimed the second European Tour title of his career by winning the International Darts Open with a 6–3 victory over Benito van de Pas in the final. Smith suffered a 10–4 defeat to Gerwyn Price in the first round of the World Matchplay, but retained his European Darts Trophy title by seeing off Michael van Gerwen in the final for the second year in a row. At the World Grand Prix he punished two missed match darts from Price to beat him 2–1, but then lost from 2–0 up in sets against Jamie Lewis in the second round. Wins over Wayne Jones, Lewis and Andy Fordham saw Smith top his group at the Grand Slam and he then averaged 104.59 to beat Dave Chisnall 10–7 and reach the quarter-finals. Smith's first major event semi-final soon followed as he defeated Lewis 16–11, but he was heavily beaten 16–6 by Van Gerwen.
At the 2016 World Championship wins over Jeffrey de Zwaan, Steve Beaton and Benito van de Pas saw Smith reach the quarter-finals for the first time, where he was 3–0 ahead of Raymond van Barneveld. He could only win two legs as Van Barneveld moved 4–3 up, but took the next to take the match to a ninth and final set. Smith finished 130 on double five to move within a leg of the match and missed one dart to win it, before Van Barneveld won four legs in a row to end Smith's event. Smith made his debut in the Premier League after receiving a wildcard from the PDC. On the opening night he played twice due to Gary Anderson being ill. Smith lost his first matches 7–1 to Peter Wright and 7–2 to Adrian Lewis. His first point came in week three courtesy of a 6–6 draw with Dave Chisnall. Michael van Gerwen scored a world record three dart average of 123.40 against Smith in a 7–1 win, but the following week Smith got his first Premier League victory by beating Van Barneveld 7–5. However, Smith lost his last two matches to finish bottom of the table and be eliminated from the event.
Smith played in the final of the 2016 Austrian Darts Open and lost 6–4 to Phil Taylor. He saw off Simon Whitlock 10–6 at the World Matchplay, before losing 11–7 to Steve Beaton in the second round and was knocked out 2–1 in sets by Alan Norris in the first round of the World Grand Prix. Smith was also eliminated in the first round of the European Championship and Players Championship Finals.
Smith entered the 2017 World Championship without a match win since September 2016 having lost 10 in a row. He took the deciding set of his first round match with Ricky Evans without dropping a leg and said he had been fighting his demons recently. In the next round he missed five darts to eliminate Mervyn King 4–1, but kept his composure to eventually see him off 4–3. Smith went 3–1 up on James Wade in the third round, but after Wade took the next set, Smith lost six legs in a row to be eliminated 4–3. He missed two darts to win the third UK Open Qualifier as Peter Wright beat him 6–5. Four 6–5 wins saw him reach the final of the Gibraltar Darts Trophy and at 4–4 he took out a 132 finish with Mensur Suljović waiting on 56 to break throw and took home his first title in nearly two years with a 6–4 victory.2012: First round (lost to Co Stompé 0–3)
2013: First round (lost to Raymond van Barneveld 0–3)
2014: Third round (lost to Peter Wright 3–4)
2015: Third round (lost to Stephen Bunting 2–4)
2016: Quarter-finals (lost to Raymond van Barneveld 4–5)
2017: Third round (lost to James Wade 3–4)